The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that the referenced prior art forms part of the common general knowledge in Australia.
Centrifugal scroll screens for dewatering of fine coal are used in a continuous process mandated by the volume demands of processes such as PCI, fluidized bed and other continuous end uses for fine coal. The general principle upon which all such screens operate is that a concentric-shaft drive assembly is used to differentially drive a frusto-conical screen assembly and a scroll assembly having one or more substantially helical scrapers within a housing.
The frusto-conical screen assembly includes a driven base (termed a “base-of-spoke piece”) closing its inner, narrow end and a screening surface formed by hoops and stringers supporting wedge wire or other screening surface-forming material. The scroll assembly typically comprises a generally frusto-conical scroll body having the one or more substantially helical scrapers fabricated thereto and having a drive flange running substantially adjacent the base-of-spoke piece.
Material to be screened is deposited by a conduit into the narrow end of the scroll body, which is provided with delivery apertures allowing the material to pass radially to the screening surface. The scroll assembly and screen assembly rotate at different speeds whereby material deposited centrifugally on the screen is urged by the relative scraping action of the scrapers on the screening surface to urge the deposited material toward the larger, discharge end of the screening assembly. The speed differential may be selected to either advance or retard the rate of flow of screened material through the apparatus, depending on the characteristics of the material being screened. The number and helical length of the scraper or scrapers may also be selected having regard to the dynamic nature of the material being screened.
U.S. Pat. No. 6,736,968 describes a typical prior art apparatus. In its fundamental particulars a horizontal centrifuge has a frusto-conical basket rotatably mounted within a housing. The basket has a plurality of rings disposed between an open, wider end and a closed, narrower end of the basket and a plurality of guide rails circumferentially spaced with respect to the rings. The guide rails have respective lengths extending between the open and the closed ends of the basket and provide a plurality of respective bearing surfaces. A tubular (frusto-conical) screen is supported in the basket by the plurality of bearing surfaces of the guide rails. A scroll with helical screw flights is also rotatably mounted within the basket. The scroll may be inserted or removed through the wide end. The tubular screen is made from six arcuate screen segments. The bearing surfaces of the guide rods and the peripheral edges of the helical screw flights are machined surfaces of otherwise simply-fabricated components. An infeed tube has an outlet extending through the outer end of the scroll and extending through the scroll body, the material passing through apertures in the scroll body wall between the helical screw flights to deposit material at the narrower driven end of the screen.
The use of six arcuate panels permits field change out by an integrated hoist system, whereby the opened housing may first have the scroll assembly changed out, followed by an outer hoop of the basket and the arcuate panels in turn. The basket may remain in the machine, have arcuate panels refitted, and forced to roundness and retained, before refitting the scroll assembly.
While this can be said to have certain advantages over the prior art, in terms of the use of lighter and cheaper fabricated components and a multipart screen surface capable of being changed out apart from the basket, it remains that there are fundamental issues of construction to address.
Firstly, the use of multipart screen sections requires high tolerance construction and requires that great attention be paid to maintaining roundness and concentricity of the screen surface on installation. Despite the assertion that using six arcuate segments provides the screen with greater circularity and allows the screen to better conform to the bearing surfaces on the guide rails, thereby making the screen more concentric with the basket and the scroll, the key issue is whether the screening surface is concentric and sized to the surface of the solid of rotation of the scroll assembly. The use of the retaining ring or clamp ring to urge the screen against the bearing surfaces, which is alleged to help maintain the roundness of the screen, instead is a high tolerance piece which is essential. The bearing surfaces of the guide rods and the peripheral edges of the helical screw flights must also be machined surfaces to provide this concentricity between the scroll and the screen, rather than being optional as implied.
Secondly, there is an inherent reduction in effective path length through having to pass the conduit into the narrow base of the scroll assembly. This actually occurs while the product flow is forced to change direction twice to output.